Identification, friend or foe

In telecommunication, an identification, friend or foe (IFF) is a system using electromagnetic transmissions to which equipment carried by friendly forces automatically responds, for example, by emitting pulses, thereby distinguishing themselves from enemy forces.

Note: The secondary surveillance radar (SSR) system used in modern air traffic control systems is an outgrowth of the military IFF system used during World War II. The IFF equipment carried by modern military aircraft is compatible with the transponder system used for civilian air traffic control.

Source: from Federal Standard 1037C and from the Department of Defense Dictionary of Military and Associated Terms